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Picture of Tube Amp Rebuild (and Mod)
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Striving for that old school sound, you purchase a 'vintage' guitar amp. But it doesn't sound quite right. Well, any amp more than 20 years old is gonna need work...

What's the deal with tube amps? Why all the fuss?

Yes, they do have a special sound, one that digital 'modeling' can't quite get right... It's actually the limitations of tubes that impart that special tone--that natural compression and smooth breakup. Sure, they keep improving solid-state amplifiers--but a look at the majority of mid-range to high-quality amps, the current models (mostly all tube, tube/ss hybrids, etc.) should convince you that it's very difficult to capture 'that sound' without tubes.

Does a 'rebuild' ruin the value of my vintage equipment?

No. Maybe. I don't know. Does changing the tubes mean the amp is not longer 'vintage?' Every old amp will need a rebuild at one point or another. If this negates it's 'vintage' status, then there is no such thing as a vintage amp! Functional amps, anyway--o.k. for nutso collectors who don't actually use 'em.... Who cares about them!

This little jem is a 1961 Kay 503A instrument (guitar, harp) amplifier. Output is in the 3-4 watt range. A nice studio, or "living room amp."

Here's a taste, and there's more video on the last step:
#1 -- Mod A (my first attempt--If you only have patience to listen to one clip, play #2):


(recognize the Trash-o-caster?)

#2 -- This is Mod B, IMHO, a much better sound:
(pickups switched from middle to middle/neck phased mix, about half way)



#3 -- Quick one added, with a Gibson Les Paul, just to show the amp has some bluesy distortion..
(pretty 'raggedy' playing on this one, but it's enough to catch the sound...)


All the videos are 'clean'--guitar and amp only, no FX.

Danger! Danger! No, really, DANGER!!!!!

A tube amp, even an unplugged tube amp, stores enough electrical energy in it's capacitors to kill you! YES, KILL YOU. Don't touch it unless you take precautions. See the section on DISCHARGE THOSE CAPS!!!!!

I'm not a tube repair technician, or an expert electrician. I'm just a hobbyist. Don't take my word, do your own research, and please be careful !
 
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Step 1: Symptoms: How do I know my amp needs a rebuild?

Picture of Symptoms: How do I know my amp needs a rebuild?
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--Hum: Usually a sign of bad filter caps. Poor routing is often a cause. Adding a three-prong cord will help.

--Weak signal: Bad tubes, failing output transformer.

--No highs/loss of tone: Bad tubes, coupling or bypass caps.

--Unpleasant distortion (not the kind you want): Often, this is bad tubes. Also could be the coupling caps or cathode bypass caps. Or could be filter caps, if the signal is modulated with 60 hz noise.

--Weird noises (squeals, crackles, etc.): Probably bad tubes. Sometimes failing coupling caps.

--In each case, look to the speaker, also. A torn or worn speaker cone could be the culprit. In rare cases the output transformer may be failing.


This amp had the following problems:

--good tone for 5 minutes, then a nasty, harsh clipping distortion on louder notes (yep, bad tubes were the cause.)

--Nasty hum (filter caps and routing problems.)

--Dull, muddy tone (needed new caps and tubes.)

--The original two-prong power cord needed replacing.

Step 2: What's involved in a rebuild?

Picture of What's involved in a rebuild?
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--A recap job
--New tubes
--Addressing safely issues

It all boils down to a lot of soldering....

Original cost: ~$100 USD.

Upgrade cost:

New tubes: ~$20
Components: ~$20-$45 (depending on what's needed.)

Parts and supplies

--New tubes

Tubes availability can be an issue. But many manufacturers continued making tubes long after they were considered obsolete. And Russian-made tubes are still produced today.

Generally, N.O.S. tubes from US or European sources are preferred (N.O.S. or "new-old stock" means: manufactured previously, but never placed in service.)

--New capacitors
--New resistors (optional)
--A terminal strip or two
--New cord/plug/fuse holder/grommet (optional)

Tools

--Soldering iron (30W--your wimpy 15W is no good here.)
--Soldering supplies--solder, desoldering tool, etc.
--needle-nose pliers
--Pop rivet tool/rivets (optional)
--Drill, dremel, etc. (optional)

--Schematics. Tacked to the inside of your amp, or online schematicheaven.com.

Step 3: Buying a vintage amp 'project'

Picture of Buying a vintage amp 'project'
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Before you buy a vintage amp, do your homework. Check for schematics here: schematicheaven. With any luck, you'll know loads about the amp before you even purchase it! (don't make a silly mistake and buy an old solid-state amp off ebay, thinking that every amp made in the '60s was tube!)

I'd recommend hands-on--i.e., play the amp before purchasing. But there are so many vintage amps on ebay, etc., that's not always possible.

Also, read some user reviews here: Harmony-central Amp Reviews

Things to look for in a vintage amp:

--Does it work at all? This could be a recipe for frustration--if the transformers are bad, you could be looking at a very expensive project (of course, you might have a bargain, too.)

--If you plan to rebuild yourself, simplicity is good. Are you prepared to tackle an amp with tremolo and reverb circuits, as well as a multi-stage amplifier circuit? If not, stay small.

--If modding is your goal, then point-to-point wiring is best (most vintage amps are wired this fashion, anyway.) Unlike a modern PCB, point-to-point is a rats-nest approach, where the components are simply tied together with lengths of wire. This is a good thing, and actually makes noise-related problems easier to fix.

Things to avoid:

--Unsafe design

Look for at least two transformers--every tube amplifier has an output transformer hooked to the speaker. But some old amps forgo input (power) transformers, and are wired directly (indirectly, actually, through a single cap) to the AC line voltage. This type of wiring lacks 'isolation' and is dangerous! If that one old cap fails, you're a conductor! (in the electrical, not the musical sense.) Check online schematics if you can't inspect the amp personally.

The Kay 503A is a bit of a hybrid. It has an isolation transformer--but only for the amplifier circuit, not the tube "heaters." This is not ideal, but it is safer than no isolation at all. With a bit of rewiring, it's not too bad. A separate power supply for the heaters would be the best option, however. Or an external line isolation transformer (replacing the AC heater circuit with an isolated DC supply will also reduce hum.)

--Check for blown speakers or torn cones.

--For a first-timer, avoid a large complex rebuilding project.

Step 4: DISCHARGE THOSE CAPS!!!!!

Picture of DISCHARGE THOSE CAPS!!!!!
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Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....

The 'filter' caps can store fatal amounts of electrical current. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.

If you're completely lost, and don't understand this, DON'T MODIFY YOUR AMP. You haven't enough knowledge to work on high voltage/current circuits safely...

Several ways to discharge caps:

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)

THEN,
--Take a screwdriver or a jumper and short the capacitors leads.
--OR jumper the power amp tube plate pin to GND for a minute or so (Class A, single power tube only.)
--OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...

Some, or all of these methods may result in a spark... Obviously, your flesh can act as a jumper also.

It's NEVER a good idea to touch amp circuitry when it's switched on. If you're stupid enough to do it, NEVER USE BOTH HANDS--that way, there's at least a slim chance the current won't flow through your heart.

And don't work on amps in bare feet, on a damp basement floor. :-(
Don't even play your amp in this fashion...


Step 5: General info about 'Recapping'

Picture of General info about 'Recapping'
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ALL tube amps need a recapping job at some point. Old power supply 'filter' capacitors are the primary cause of hum. And used or not, the capacitors fail over time. Electrolytic (tubular) capacitors have an optimum life of 10 years. Of course, they may last much longer. They will often operate sub-optimally for much longer.

But tube gear is sensitive to cap failure. Tubes circuits operate at higher voltages, and many amps are quite simple. Older caps with outmoded technology ('paper electrolytic') are almost certain to fail over time. In one sense, tubes are less susceptible to cap failure--unlike solid-state amps, many will continue to operate, although poorly.

Cap types

Most common replacement caps are:

Non-polarized (polypropylene, or mylar)
Rating: 500V minimum

Most builders (and rebuilders) use high quality polypropylene ('orange drop') caps. But a quality mylar cap exceeds the characteristics of the original capacitors. I wanted to retain the vintage sound, so I choose mylar. It's amateur builders, not pros, that insist a certain type of cap is a must. In any event, the difference is subtle, and the qualitative differences are a matter of personal taste.

Is the difference between cap types real? Probably.
Here's an interesting cap bench test.

Polarized (electrolytic)
Rating: depends on the amp. I used 350V caps where the originals were 150V.

Regular 'computer' type electrolytic capacitors work fine. Especially for the power supply filter caps.

They come in two packages: radial, with the leads on the same end, and axial, with the leads on inline on both ends.

Oh, Yeah...as the name implies, polarized caps must be installed with the correct....polarity. Inserting one backwards--fireworks!

Note: The first set of filter caps installed were purchased at a local electronics store. This store is nerd heaven--or so I thought. Some of their components are in yellowing packaging; clearly getting old. I picked what looked like the newest of the bunch. The caps (40uF, 350V) lasted for two weeks before failing--first a bit of crackling, then a lot of crackling and the amp began cutting out....

From now on, it's mail-order from a reputable firm (digikey, jameco, etc.)

Step 6: Replacing the filter 'can capacitors'

Picture of Replacing the filter 'can capacitors'
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Many guitar amps have the larger electrolytic caps in sealed "cans." Quite a few, this one included, have multi-capacitor cans--more than one capacitor together in a single package. They usually are the power source filter caps.

These can be replaced, but--

--Replacements are 5-10X as expensive as a handful of single caps
--Even if you can find an electrically-identical replacement, finding the same physical dimensions is difficult (height, diameter, etc.)
--Fewer options with cans. If circuit changes are made, and not all the sections are used, it's just a waste.
--You don't always know if you're getting new cans or 20 yr old 'N.O.S' ('New Old Stock'--good for tubes, bad for caps.)
--Even brand-new caps have a failure rate. Much less painful to replace one $7 cap than a $40 "can" type...

So it's quite easy and common to replace with individual polarized capacitors. I removed all the wiring from the can, then rerouted to the new caps with a terminal strip. Terminal strip are cheap, simple to use, and very common in vintage gear.

The terminal strip was attached to the chassis with a steel pop rivet and steel rivet backing-plate. The hole was pre-existing (cool!) but drilling a new one would be easy.

The old 'can' was left in place for looks, and for a reference to whoever 'recaps' this amp in the future.

The filter caps, originally rated at 150V, were replaced with 350V versions. Yes, you caught me--the 20 uF bypass cap is still only 160V, because that was the only one I had on hand (This is part of Mod A--I've since replaced it with a 10uF 350V cap...)

When replacing the filter caps, don't exceed the capacitance values by much (I went from 40 uF spec to 47 uF.) More capacitance sounds good, right? Less hum, you say? Unfortunately, amps with tube rectifiers can't handle high capacitance values--they cause voltage spikes that wear out the rectifiers quickly. 60 - 100 uF is the recommended max for this type of rectifier...stay near the spec values here.

This link has a tutorial on encapsulating (hiding) new caps inside an old cap can:
http://www.nmr.mgh.harvard.edu/~reese/electrolytics/

Step 7: Replacing 'the other' caps

Picture of Replacing 'the other' caps
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Other than the filter capacitors, the other caps in the amp will be one of these (Step 8 will help you identify each of these types) :

1) Coupling Caps -- connect the signal path between stages
2) Tone Caps -- roll off different frequencies to ground, for tonal effects
3) Cathode-Bypass Caps -- integral part of "cathode biased" circuits

Most of these caps will be of the non-polarized variety. The occasional cathode bypass cap could be a polarized, however.

The coupling and tone caps should have voltage ratings comfortably higher than the max voltage the circuit might see--with cap and induction spikes, just assume these should be 400 volts or greater.

The bias voltage for tubes rarely exceeds 10-15 volts, however. A 50V capacitor will work fine for the cathode-bypass caps.

Use a 30W soldering iron. Grasp the leads of the old caps (with needle-nose pliers, if you are smart), and apply the iron. They may take some wrestling to remove. If you can avoid cutting the lead, so much the better.

Pulling vs. cutting usually leaves you a hole to insert the new replacement cap. Finagle it in, solder and you're done....

Many people would say I'm a bad boy for not using 'orange drops'....(seriously, the amp sounds much better than before, with the new mylar caps..) I do expect to be criticized for not using them, but the majority of professional amplifier technicians say that the differences are subtle and a matter of taste...

Still, remember these are the caps that the audio signal passes through. So avoid the cheapest ceramic caps, for instance.

Step 8: Mod that amp!

Picture of Mod that amp!
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A rebuild is the perfect time to mod your amp!

Here's a great link: Tubes for Dummies! :-) It outlines how each component in a tube circuit functions, and explains their purpose. Note: designing viable circuits really is more complicated than this...

Search for schematics of similar amps. I found at least 7 schematics of amps that use the 12AU6, 35Z5 and 50L6 tubes. Compare those schematics to your amp. Having similar examples is the best way to start, especially if you are unfamiliar with tube circuitry.

Note that the schematic drawing for the 12AU6 looks different--this is a pentode valve, rather than a triode, which is normally used for preamps. But the component functions are analogous.

Make one change at a time. Really large changes will probably be...unsuccessful (unless you really know what you're doing, or the design was flawed to begin with.) Here are some examples:

--Change coupling caps. They'll effect the tone. Smaller values will be brighter, larger values will add bass.

--Adjust the " grid leak resistor ," which effects the impedance and the gain of the stage.

--The initial input attenuation/RC network can be adjusted (resistors directly after the jack.)

--Cathode bypass caps on both the preamp and the power amp stage can be adjusted. This effects both the response and the tone (to a lesser extent.)

--Changing the values of the Plate Resistor and the Cathode Resistor have a direct effect on the gain, headroom and distortion. Don't do this without a thorough understanding of the tube's characteristics, however.

--Changing your speaker is another modding possibility. Be sure to match the speaker's impedance to the transformer.

--Output transformers are harder to replace, unless the schematic is well documented. If it's not, look up the power amp tube data sheet and it's 'load resistance.' For instance, load resistance for the a 50L6 @ 100V is 2000 ohms, and @ 200V, 4000 ohms. This amp uses a plate voltage of 135V, so 2500-3000 ohms for the primary is close. Choose the secondary impedance based on the speaker...

Step 9: The 'Base Mods'

Picture of The 'Base Mods'
These modifications are common to both Mods A & B...

--The grid leak resistor on the preamp (12AU6 tube.) Other similar designs for a 12UA6 preamp fell in a range from 2.2 Meg to 15 Meg. The Kay 503 had a 3.9 Meg resistor--I substituted a 15 Meg. This gives the preamp perhaps a bit more 'gain.'

--Input Stage: I removed the 22K resistor tied to GND here, as it wasn't wired correctly, anyway (it was wired directly to GND before the series resistor--effectively making the circuit for inputs #2 & #3 different from #1. Unless that was the plan, but it's not on the schematic....)

One input jack was removed. NO ONE EVER plugs in more than one instrument (unless they don't care about audio quality.) Two inputs jacks were plenty--each one slightly different, more room to work, and less noise. One is wired direct to the coupling cap, the other through a 22K resistor.

A one meg resistor to ground was added for higher input impedance. In this configuration, it's identical to using a one meg POT (at max) as an input volume control.

--The filter cap replacement (See the page Replacing the filter 'can capacitors.' )

--Grounded plug mod (see the page Three-prong plug safety mod.)

Now, on to the real mods....

Step 10: First time around--' Mod A'

Picture of First time around--' Mod A'
After plugging in a few alternate components, I liked the setup I call Mod A.

While it has a bit more sting than the the original wiring, it's also fairly clean-sounding.

Significant changes:

--Substituted a 0.1 uf bypass cap for the preamp stage. I thought this would make the signal 'muddier,' but the extra bass added something to the sound.

--Used a 20 uF cathode bypass cap for the power amp stage. This changed the response to more treble, and together with the preamp alterations help shape the sound.

The 'Mod A' circuit has a real 'bell-like' tone.

Step 11: 'Mod B'--the next step

Picture of 'Mod B'--the next step
Mod A was pretty good, but I still wanted a bit more 'crunchy' from the amp. Mod B is my best effort (yet.)

Mod B has the following improvements:

--Following the example of Silvertone amps with similar tubes (1430, 1448, etc.) the first coupling cap was changed from .05 to .01uF, resulting in a good boost to the high-end.

--Along the same lines, the preamp cathode bypass cap was changed to 0.022 uF (smaller cap values here tend to lessen bass response.)

A second, larger cap (0.068uF) was added with a cut-out switch, so a fuller, jazzier tone can be added at will. The difference is subtle, but noticeable. I find it's difficult to be objective about the tonal effects of cathode bypass caps, but changes seem to alter the response, or 'tightness' of the attack, and to a lesser extent the overall tone...as though the note 'envelope' itself were subtly changed.

--The preamp-to-power-stage coupling cap was changed from .005 to .001uF. Again, more high-end boost.

--The volume control POT upped to 1 Meg (from 500K.) Since it also functions as a grid leak resistor for the power amp, this extra resistance ups the gain. Well, less signal attenuation, anyway. The additional distortion is noticeable...

Replacing with a 2 Meg POT, or moving the volume control to the preamp are other options for punching up the power stage.

--The power amp cathode bypass cap is now 10uF, further enhancing the high-end response (or actually, reducing the bass.)

Step 12: Routing, and other tips...

Picture of Routing, and other tips...
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Routing

This turned out to be significant for my amp. The tone signal wire was routed beside the filament heater loop (the wire had a funny kink in it, which indicates that it was originally routed elsewhere.) The yellow wire in the photos is the heater circuit.

Signal wires should be separated from the heater circuit, especially if it's an AC loop, like this one. It's a prime source of hum.

Any long reaches, such as output to the transformer should be in twisted-pairs. A twisted-pair signal wire performs nearly as well as a fully-shielded cable.

If the hum persists, perhaps a ground-loop is at fault. Point-to-point wiring should utilize 'star grounding,' in which loops are avoided.

Oh, oh, Don't try this at home, Kids!:

One way to chase down hum is to move the components around while the amp is live. A wooden stick can be used to adjust position.

Don't use a pencil! Don't cross wires and short them! If you're smart, don't do this at all....

Other tips

--Testing with the speaker in the cabinet has a significant effect on the sound. An unenclosed speaker will have more treble (and likely not be as loud, too.)

--While on the subject of speakers, be very careful with screwdrivers, etc. around the speaker(s). Depending on the size, a new speaker will set you back $30 to $300 USD (reconeing is another option, but better to just avoid the issue.)

--Connecting two speakers in parallel reduces resistance, increasing the load. If both speakers are 4 ohm, then the load becomes 2 ohm! This is perilously close to a 'hard short.' A tube power amp can probably handle the current, but if you're messing with a transistor power amp, beware!

Even if the amp circuitry survives, there's a good chance you'll blow the output transformer if the speaker isn't close to spec...

Step 13: Three-prong plug safety mod

Picture of Three-prong plug safety mod
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If your amp doesn't have a grounded plug, then this mod will make it both safer and quieter. I reused a computer cord. This hasn't completely isolated the amp from line (the heater circuit mentioned in previous references), but it should be safe enough to use.

A grounded AC plug (in the USA) has the following color codes:

--BLACK : Hot
--WHITE : Neutral
--GREEN : Ground

Route the black wire through a fuse holder to the power switch. The white wire is connected directly to the transformer. Connect the green GRD to the chassis ground. This means screwed or riveted directly to the chassis--not just soldered to an existing grounded wire.

The fuse holder (and fuse) was added, not being OEM for this amp. So a 1/2 hole was drilled for mounting.

A larger rubber grommet replaced the original cord grommet. The 'stress relief' consists of two zip ties, and could be better...

Here's an in-depth explaination of the mod: http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html

For folks outside the US, the international power cord color codes:

--BROWN : Hot
--BLUE : Neutral
--GREEN/YELLOW STRIPE : Ground

Step 14: Final thoughts

Picture of Final thoughts
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I'm very happy with the 'new' sound, and the safety improvements on the amp. It's a nice 'living room' and recording amp. The amp is brighter and a bit 'punchier' than it was originally. It has a real bell-like tone...

From the video clips, it's obvious that the amp is now very quiet from a noise perspective (well, for Mod A, anyway)--sorry, no "before" video to compare to... For an amp that's only 3 watts (perhaps a bit more with fresh tubes), it's louder than you'd think.

While the Mod A upped the treble and punch, and Mod B got some of the the 'garage' tone I was hoping for, I'm not completely satisfied... If more 'drive' is the goal, the single stage preamp would probably limit the smoothness....And if radical changes were made--well, better to just build one from scratch....

Would I do it differently next time? Yes.

I'd tear the amp down to the bare chassis and rebuild from there. But what began as a simple 'filter cap' recap job sort of snowballed...

I'd certainly clean up the interior, tidy the wiring rats-nest, remove the dust, etc. Probably go the extra mile and use the 'orange drop' caps.

Since this isn't a highly desirable amp like an old Tweed Champ, the materials used aren't going to tick-off any collectors. The design itself utilizes common radio tubes, so even N.O.S. tubes are cheap and plentiful. I can experiment to my heart's content.

Anyway, I learned something, and I'll know better next time.

More video/audio
All examples are 'clean'--just guitar and amp, no F/X.

#4 -- Mod A, again with the Trash-o-caster (the video cam doesn't quite capture the treble correctly):



Obviously, the sound quality of a small digital camera (functioning as a vid recorder) isn't going to be excellent...but it's good enough to get an idea...



Aesthetic disclaimer

Everyone's taste in sound is different. it's a personal thing. So before you write:

But my Supratronic Billy-Bob-Mcgee Boutique Amp sounds better than this!
or
Your mods suck, it doesn't sound like a (insert Marshall, Fender, Vox, etc. here)!
or
That sucks, it doesn't sound 'metal' at all!
or
HAHAHAHAHA, my Whooptytron Jimiaxeulator is 100000 watts! What good is this???

Well, you can't make a full-sized SUV from a go-cart chassis (but both are cool!) Any tube amp can be modified and the sound shaped, but it's basic character will not change. This is a single-stage preamp, Class A tube amplifier.

It's really a cool-sounding amp. But I understand if it's not your cup-o-tea...

And I have other amps. If I want a different sound, I'll use them.
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ksmith1351 year ago
Thank u
ksmith1351 year ago
Hey gmoon. Do u have any info on an Ampeg vt60 combo. I'm trying to bring it back to life and I'm having a hard time. Thanks.
gmoon (author)  ksmith1351 year ago

No direct experience. I know there's a VT120 schematic on Schematic Heaven, which is probably identical to the VT60, other than having double the output tubes...

ksmith1351 year ago
Hey gmoon. Ur helping me a lot.
gmoon (author)  ksmith1351 year ago

Cool. Be safe with this stuff...

DANIELEPO6 years ago
hi! I'm thinking of modding my amp and found this very helpfull. only one question: what you mean with high-end? I thought lowering those caps would increase the bass... thanks for all
gmoon (author)  DANIELEPO6 years ago
Hey, good question. I assume by saying "lowering those caps," you mean make smaller values, right?

General rule is that the smaller the cap value, the more it blocks low frequencies.

(jeeze, I see that I have several duplicate component names-- two C3, two C2, etc. Bear with me, I hope this makes sense with the lousy schematic..)

How that works to filter frequencies depends on how it's oriented in the circuit:

-- caps inline with the signal, like the 0.01uF (upper most cap labeled C3), are "high pass" filters. They block low frequencies from passing forward. These are called "coupling caps", and also block any DC voltage offsets from messing with the next stage.

Changing from .05 to .01 blocks more lows from passing.

-- caps that shunt or bleed signal to ground, like the 0.001 that's attached to the tone POT (lower C2) are "low pass" filters. Those caps block low frequencies from passing to the ground--which means the the high frequencies are now escaping away from the signal path.

That's the way a passive guitar tone control works--the cap cuts the high frequencies by letting them pass through to the ground.

So in each case, small caps block more lower frequencies. It's just a matter of how they are used. Here's a simple calculator and illustration of high vs. low pass.

The other caps are cathode bypass and control grid caps for the tubes, and they're a little more tricky to explain...
DANIELEPO gmoon6 years ago
Thanks for you very good explanation, now the hole thing is more clear for me! Now comes the hard question. I have an amp that's almost the same as yours, it's a 803c, and because I'm a harp player I'd like it to be more "warm" (less bright). I found a lot of mods to warm the sound but no one tells you why the amp warms up. So, here is the question: what could be the frequency response of a "warm" amp? I know that the asymmetric distortion of the tube is also important obtaining "that sound", thats why I bought that type of amp, but I'd like to understand better the other variables. thanks for all
gmoon (author)  DANIELEPO6 years ago
It really difficult to quantify something like "warmth," since everyone has a different idea of what that means.

One thing tubes do is color sound with even-order (mostly second-order) distortion. It's a type of distortion closest to the fundamental sound itself, and is generally found to be pleasing to the ear. Mosfets can do this too, but somewhat differently.

But a good guitar tube amps also have odd-order distortion (third, fifth, etc.), which is the "bite" or "crunch" we love.

Frequency response itself is interesting, since a guitar (or harp) amp doesn't reproduce sound faithfully--as a stereo system they sound like crap. Plus a guitar speaker normally doesn't have a frequency response above 5kHz or so. But you will hear higher frequencies--they come from the speaker cone and the cabinet / baffle board vibrating at higher harmonics.


Just to clarify, when I wrote: small caps block more lower frequencies, that's a little misleading.

Caps block frequencies below the cutoff freq. Smaller caps don't block more lower frequencies--the cutoff frequency moves higher, so more mid-range cut is added to the already blocked frequencies below.

In the chart here of a high-pass filter, the cutoff would move to the right, as the cap value gets smaller.
It will become clear if we look at the formula for the REACTIVE resistance of a capacitor the formula is Xc=1/2*3.142*frequency* copacity IN FARAD Or as we say it one over two pie eff see Here you can see that the higher the frequency the lower the effective resistance and the larger the capacitor the lower the effective resistance
Ceiling cat5 years ago
Hi.  I was wondering if you could help me.  I have an old Lindell three tube amp and it's not working properly.  When I turn it on, as soon as the tubes warm up there is a VERY LOUD hum , and it's unaffected by the volume dial.  Could this be a bad filtering cap?
Yes bad filtering caps are usually the cause.
gmoon (author)  Ceiling cat5 years ago
Hi. Older amps almost always need new filter caps at some point. Loud hum regardless of volume does seem to point in that direction.

It could be other things, too, though. Sometime the tubes themselves develop hum and have to be replaced.

You could try pulling the preamp tube and then turning on the amp--unless it's a "radio tube" type amp that has all the filaments wired in series (like the amp in this instructable.) In that case the amp won't work with one tube pulled.

Always nice to have a few backup tubes anyway, so new tubes are good investment...
Thanks. I'm actually on my way out to Radio Shack for new caps.  As for the tube  configuration, I think it is how you said.  It's a very simple circuit.  And don't worry, I have plenty of tubes around.  I bought a bin of four tube radios and a box full of tubes at a swappers meet for $10.  Thats where I got the 12AU7 for my tube boost pedal.  I have like seven.
gmoon (author)  Ceiling cat5 years ago
Ok, cool, you've eliminated tubes as a problem.

Good luck finding the caps at RadioShack, though. The store near me doesn't carry electrolytics at the capacitance & voltage needed for a tube amp...(even for the relatively low voltage of "radio tube" amps-- 120V-150V.)
Unfortunately I found out it's the same story here.  I biked a few miles to RadioShack only to find they didn't have the caps I needed.  Not even the right capacitance.  It wasn't a total loss, though.  I got some electrical tape and solder that I needed badly.  And I got a ride home with a friend I ran into.
Oh, and earlier today I found a fuse holder with a mount for free.  I'll have to do the fuse/three-prong-plug mod sometime.
Good news!   Last week I bought some new caps at a local amp repair shop called "Mike Tech".  I soldered them in and took it back there and he formed them with a variac for free.  I brought it home, hooked it up, and the results were great.  It sounds beautiful!  It's got a great tube sound when I crank up the volume.  Very happy with it, and glad my dad's old amp is still in the family and finally working and being played again.
hg3414 years ago
my grandpa was move out to a smaller house and geting rip of most of his stuff a few years back and i took some thing that look like a ham raido

i hadnt realy look at it since then in till last weekend and found out in was a 300w tube amp by eico

im trying to get speakers for it now
gmoon (author)  hg3414 years ago
Pretty cool. Should be worth fixing up.

(don't run it without speakers...)
hg341 gmoon4 years ago
im sry i get all execited and grously over-estamate what some things can do....
its only 14 watts

buts its the famed hf-81

every review i saw was like this

http://www.stereophile.com/integratedamps/606eico/
gmoon (author)  hg3414 years ago
Definitely worth fixing up.

12-14 watts of tube power is LOUD. Loudness is logarithmic--subjectively, twice as loud as 14 watts is 140 watts.
hg341 gmoon4 years ago
on the back of it it has screws for the speakers marked g 4 8 16 and 32
what dose this mean(i think it the ohms but i dont know...) i do know what g is...
1st is what im talking about 2nd is a bouns
IMG_1945.JPGIMG_1947.JPG
cgillyard hg3414 years ago
That is your speaker impedance terminal strip. Use them to connect your speakers to the corresponding impedance screws. For example if you have a set of 8 OHM speakers. Connect the positive (+) terminal of your speaker to the 8 OHM screw and the negative (-) terminal of your speaker to the 0 screw to the amp. Sometimes that 0 terminal screw on your amp is also known as C or common. Hope this helps. Let me know if anything. Thanks.
gmoon (author)  hg3414 years ago
Yeah, that's ohms. For an 8 ohm speaker, connect one wire to "8" and the other wire to "g" (ground).

Must be stereo, so two speakers (at least) are required--one per channel.

1st is what im talking about 2nd is a bouns

Sorry, I didn't get that...
hg341 gmoon4 years ago
ok im looking to buy some speakers for this what would be best 2 8 16 or 32?
gmoon (author)  hg3414 years ago
None is inherently better; all the better speakers come in different ohms loads. Just work with what's available.

You can also wire speakers together. Two (identical) speakers in series doubles the resistance; Two in parallel halves the resistance...
hg341 gmoon4 years ago
cool thanks for that

"1st is what im talking about 2nd is a bouns" i was talking about the pics
the 2nd one is the uncoverd bottom of my amp no real need to show but its still fun to look at.
i was wondering, what kind of output you could get from just one tube? I've been thinking about making a tube headphone amp but haven't seen any schematics anywhere (maybe i haven't looked hard enough), and i wasn't sure whether it would be plausible...maybe just a tube preamp and if the signal is still too weak a little bit of solid state amplification (i heard FET's sound more "tubie" then most opamps)...i should probably just look at some schematics and see how they manage the headphone jack
gmoon (author)  carpe_noctem4 years ago
A single tube is a little problematic, especially if you're looking for a classic tube sound.. A hybrid approach has been done--tube preamp with an FET or a LM386 to drive the headphones would work.

The other possible approach would be to use triode/pentode paired in a single tube. There are quite a few out there. You might even try one of the low-voltage tubes intended for cars and portable radios. This page has some possible combo tubes... These are safer to use than normal high-voltage tubes.
thanks
s3mamill4 years ago
I just got this exact same amp. I thought it would be a good starter project since its a simple circuit, but it seems over my head, since I have very little experience with electronics, wiring diagrams, etc. Now I'm considering some options: 1. Have it serviced by a pro. It would be safer and the work would be done right. Low risk, but higher cost. 2. tearing out what's there and installing a kit in the body. Higher cost, but doing the work myself. 3. Trying your mods and risk messing it up irrevocably. Low cost, but high risk.

When you said "I'd tear the amp down to the bare chassis and rebuild from there." did you mean rebuild from a kit or design a circuit yourself? Thanks for a great instructable!
DSCF9683.JPG
gmoon (author)  s3mamill4 years ago
Hey, that looks good.

By "tear it down and rebuild," I meant that a less "hacked" approach would probably lead to a cleaner, more organized, less cluttered circuit. As-is, it's gonna be more difficult to replace the caps in 30 years. The tangled wiring is more fragile than a well-planned strip-down / rebuild, too.

However, it's extremely quiet and sounds great, so I'm not too disappointed.

I'm not sure what approach is right for you. If you try rebuilding it yourself (and fail), you can always take it in to a profession to get it sorted. Just be REALLY CAREFUL if you do.

If you're not confident you can do this safely, take it to an amp tech...
Man, it's crazy that you posted this because I was just wondering why my old Gibson Falcon amp shocked me when I was barefoot! How long do the filter caps hold a charge?
gmoon (author)  FireandIcebike7 years ago
Caps can hold a charge for a loooooong time (although the older ones usually don't, 'cause they're failing.) Were you pokin' around inside or just playing? Isolation can be a problem with these oldies, too.

I've been dumb enough to play my old Ampeg in the basement with barefeet....wasn't a painful shock, I could just feel the current flowing from my left hand to the floor. One lesson learned.
I was just playing. Man, I didn't think an amp could be all lethal and junk! Thanks for the info.
gmoon (author)  FireandIcebike7 years ago
Yeah, actually quite a few people have died from this stuff. Very likely your guitar is wired to use you as a ground (through the strings.) On some old amps, the chassis ground might instead be a lethal voltage in respect to another 'ground' (you in your bare feet, or a mic stand, for instance.) There's a little info on Step 3 about that, if it's an amp without an isolation transformer. But even the amps with isolation can be dangerous--that's why I added the three-prong cord here....
gmoon (author)  gmoon7 years ago
Here's an illustration of an older amp with an isolation transformer that still dangerous-- This is part of a schematic of my Ampeg Gemini II.

Because AC line plugs were all two prong, the amp has a two-position standby switch. This switch not only works as a standby, but it also connects the AC line to the chassis through a single capacitor.

Since the plugs weren't polarized, you could use this to connect the chassis ground indirectly to the line 'neutral' wire, and your amp would run quieter. But if the cap degrades and fails, and your guitar strings are grounded (as most are)--it's like poking your finger in the wall socket...

On cheaper amps without a standby switch, the cap was still there, but you'd change the plug direction to ditch the hum...

(picture: red square shows the switch and the 'death cap.')
g15_deathcap.jpg
 Shocking to say the least, especially when I'd be playing my guitar then would touch my friends at the same time...YIKES!!!!  that GeminiII....
gmoon (author)  satanalingus5 years ago
That Gemini cured me of ever playing guitar in bare feet...
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